In nuclear power generation, a graphite fuel element block typically contains a multitude of fuel channels each containing 15 or more fuel compacts stacked end-to-end to form fuel rods within the graphite block. Each fuel compact, in turn, typically contains thousands of U-235 isotopically enriched uranium ceramic fuel particles which, through fission, generate heat. In some reactor designs, uranium 238 or thorium 232 ceramic particles are also included. These convert to plutonium or uranium 233 respectively which are fissionable. Other elements can also be incorporated for neutron absorption control or production of special isotopes such as tritium. The designation "ceramic" refers typically to oxide, nitride, carbide, oxycarbide or other high temperature compounds.
To impart strength to these fuel particles and to retain fission products during normal (and sometimes abnormal) operating conditions, each individual particle is coated with a variety of products such as silicon carbide (U.S. Pat. No. 3,776,759 to Ford et al.), a pyrolytic carbon compound (U.S. Pat. No. 3,945,884 to Freck) or a combination of the two (U.S. Pat. No. 3,249,509 to Blocher, Jr., U.S. Pat. No. 3,276,968 to Ingleby, and many others).
A current fuel compact manufacturing specification calls for a 95% confidence level that no more than 5 particles out of 100,000 have a coating defect. This level of quality control requires extensive sampling and destructive examination of the compact and/or particles to assure compliance.
It is thus an object of this invention to provide a means of achieving the above level of quality and confidence. It is another object of this invention to provide a final coating that can be subjected to a quick, non-destructive method of inspecting the fuel compact in order to assess whether this compact complies with the required specifications or not. A further object of this invention is to provide an additional barrier of silicon carbide so as to contain fission products released during operation or postulated accident condition. Another object of this invention is to enable the silicon carbide coating thickness to be varied as needed under the expected conditions. Still a further object of this invention is to provide a means for curing any defects should such be found. These and other objects of this invention will become obvious upon further investigation.